Wideband high efficiency transmitter system



p 1967 J. R. BOYKIN' 3,343,088 I WIDEBAND HIGH EFFICIENCY TRANSMITTER SYSTEM Filed Dec. 12, 1963 TRANSMITTER f o A2 w AR WITH IMPEDANCE N Z Q ZQ CONTROLLED INVERTER SYSTEM Fig.|. v. LOW OUTPUT IMPEDANCE Y 39 2 23 I4; I A T L souo STATE 2 2| 3o 32 I as 25 3a 4 Fi 2 TRANSMITTER L 1 34 42 9a LO-PASS FILTER 25 e2 ""?"I(I"'K" 4s 72 5 76 I 7 TVs I 63 L A224 I 22 WITNESSES INVENTOR MW John R. Boykin United States Patent O 3,343,088 WIDEBAND HIGH EFFICIENCY TRANSMITTER SYSTEM John R. Boykin, Glen Burnie, Md, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 12, 1963, Ser. No. 330,058 7 Claims. (Cl. 325-105) This invention relates to radio frequency transmitter apparatus and more particularly to a means for increasing the bandwidth capability of a transmitter system operating at medium, low, or very low frequencies into an electrically short, narrow band antenna.

At medium frequencies, low frequencies (LF) and very low frequencies (VLF), economical and practical considerations often force the use of single vertical transmitting antennas that are electrically short in length compared to a quarter wavelength of the transmitted signal. Moreover, when the antenna is series-fed, the resulting impedance as seen looking across the base insulator can be approximated over a considerable frequency range by a series circuit composed of a resistance, an inductance and a capacitance with the impedance of the capacitor reactance being many times higher than the resistance at the frequency of operation. One approach often used to excite such an antenna is to connect in series with it a loading coil with an inductive reactance equal to the difference between the inductive and capactive reactances at the desired operating frequency. This resonates the antenna causing the combination to appear as a resistance at that frequency. Due to the fact that the reactances are high and the resistance low, the result is a highly quality factor, or Q, which is a figure of merit equal to the reactance of the antenna divided by its resistance. When such a circuit is driven from a transmitter of conventional design, the modulation, whether it be amplitude, frequency, or phase modulation, is limited in information handling capability because the transmitter will not be properly loaded at the sideband frequencies due to the high Q which results in relatively narrow bandwidth with a steep slope to its frequency response characteristic.

There have been many methods used to broadband such antennas when driven at the frequencies presently of interest. One method is to operate on the antenna by adding top loading and shunt feeding, etc. This is expensive and limits the flexibility of use at different frequencies. Another method is to use a coupling network that is essentially a doubled tuned circuit near critical coupling. To obtain this critical coupling, the primary circuit must be damped either by the transmitter source impedance or the combination of the transmitter source impedance and added external resistance. This usually results in the overall efficiency from power supply to antenna being quite low, such as 50% or less.

It is an object of the present invention therefore to provide a means to increase the bandwidth of a transmitter system operating in the medium to very low frequency range while maintaining high efficiency,

Still another object of the present invention is to broadband a transmitter system operating into a narrow band antenna while maintaining high efficiency.

Still another object of the present invention is to improve the information handling capability of a narrow band antenna in the very low frequency and low frequency ranges.

It is yet another object of the present invention to provide a means for broadbanding a solid state transmitter system operating in the VLF and LF frequency regions.

Briefly, the subject invention comprises a transmitter having a controlled output impedance working into a narrow band antenna system, i.e., an antenna which is elec- 3,343,088 Patented Sept. 19, 1967 trically short in length with respect to a quarter wavelength of the frequency to be transmitted. Additionally, an impedance inverter network is coupled between the transmitter and the antenna system so that the current in the antenna will be essentially in quadrature with the current drawn from the low impedance transmitter. The impedance of the transmitter is controlled so that the output impedance looking back into the transmitter is very low compared to the load impedance, comprising the circuit combination of the impedance inverter network and the antenna system. Furthermore, the output impedance of the transmitter is controlled so that it is a definite predetermined fraction of the load impedance.

Other objects and advantages of the subject invention will become apparent as the following specification is studied when read in connection with the accompanying drawings, in which:

FIGURE 1 is an illustration of the subject invention in block diagrammatic form;

FIGURE 2 is a schematic diagram illustrative of one embodiment of the subject invention;

FIGURE 3 is a more detailed schematic diagram of the embodiment shown in FIGURE 2;

FIGURE 4 is a schematic diagram of another embodiment of the apparatus disclosed in FIGURE 2;

FIGURE 5 is a schematic diagram of yet another embodiment of the apparatus disclosed in FIGURE 2;

FIGURE 6 is a schematic diagram of still another embodiment of apparatus disclosed in FIGURE 2; and

FIGURE 7 is another embodiment of an impedance inverter network illustrated in FIGURE 2.

Directing attention to FIGURE 1, the inventive concept is shown in block diagrammatic form wherein a transmitter with a controlled relatively low output impedance 10 operates into a narrow band antenna system 14 through an impedance inverter network 12 for reasons which will become hereinafter more evident. The controlled output impedance of the transmitter 10 is such that it is low compared to the load impedance into which the transmitter operates, which load impedance comprises the combination of the impedance inverter 12 and the narrow band antenna system 14.

FIGURE 2 is an illustrative embodiment of the subject invention wherein a solid state transmitter It) having output terminals 21 and 22 is coupled to an impedance inverter network 12 shown for purposes of illustration as a T-network comprising inductors 30 and 32, and a capacitor 34. Inductor 30 is connected to output terminal 21 while inductor 32 is connected to terminal 23. The capacitor 34 is connected to the common connection between inductors 30 and 32 with its opposite end connected to output terminal 22. Terminal 22 is returned to a point of reference potential illustrated and hereinafter referred to as ground. The side of the capacitor 34 connected to output terminal 22 is likewise connected to terminal 24. Terminals 23 and 24 become the common terminals between the impedance inverter network 12 and the narrow band antenna system 14. The narrow band antenna system 14 is illustrated in equivalent circuit form comprising the series combination of resistance 42, capacitor 40 and the inductor 38. It should be noted that the value of the inductance 38 is relatively small compared to the capacitive reactance of capacitor 40. This is inherent in a short antenna, i.e., one which has an electrically short length compared to a quarter wavelength of the frequency with which it is designed to operate.

A typical example of a short antenna in the frequency region around 50 kilocycles (kc.) would be one in which the inductive reactance X =+j ohms, the capacitive reactance X =j 200 ohms, and the resistance R=2 ohms. These representative values can for purposes of illustration be applied to the inductor 38, the capacitor 40 and the resistance 42 respectively. It becomes immediately evident that the capacitive reactance X is greater than the inductive reactance X As is the usual practice, a loading coil 36 would be coupled to the antenna to make it resonant, i.e., make the inductive reactance X equal to the capacitive reactance X at a predetermined frequency. The loading coil 36 then would be made to have an inductive reactance X equal to 100 ohms so that the impedance seen looking into the antenna system 14 from terminals 23 and 24 would be 2 ohms of resistive impedance at the frequency of 50 kc. As the frequency is increased, the impedance rises and becomes inductive, and as the frequency is reduced, the impedance rises and becomes capacitive. In order to keep constant power into the antenna, the voltage applied at the terminals 23 and 24 would have to vary in a like manner with respect to frequency.

The use of impedance inverters is well known. One type of impedance inverter is a quarter wave transmission line having a surge impedance or characteristic impedance equal to the resistive impedance of the antenna system at resonance. The surge impedance is the socalled iterive impedance of an infinite number of finite sections and determines the amount of current that can flow when a :given voltage is applied to an infinitely long line. Furthermore, the input impedance of a quarter wave transmission line is inductive at frequencies on one side of resonance, capacitive on the other side of resonance, and resistive at theresonant frequency where the resonant frequency is that frequency at which the length of each conductor is a quarter wave. As the frequency is varied, it will be found that the quarter wave transmission line acts to invert the impedance due to the fact that as frequency is increased, the impedance will become. lower and capacitive, while if the frequency is reduced, the. impedance will be reduced and become inductive.

If the ratio of the source impedance of the transmitter to the load impedance facing it is properly chosen, the extra current drawn from the transmitter by the quarter wavelength line will substantially provide the extra voltage needed on the antenna to keep constant antenna current. The use of an actual quarter wavelength transmission line at low frequencies is cumbersome; however, an artificial transmission line such as a T-network having a 90 degree phase shift may be substituted. FIGURE 2 illustrates such a configuration as the impedance inverter 12 comprising the inductors 30 and 32 and the capacitor 34. The T-network shown in FIGURE 2 is similar in connection to an ordinary T network, however, the inversion lies in the selection of component values so that the phase shift is 90 degrees.

It should be noted that the T-network is shown by way of example only and is not meant to be considered in a limiting sense, for example, the T-network illustrated as the impedance inverter network 12 could be replaced by a 1r network which provides a 90 degree phase shift. Although it doesnt make any difference mathematically whether the 90 degree phase shift is leading or lagging, it has been found that as a practical matter a 90 degree lagging phase shift is preferred. Other types of 90 degree phase shift networks which can be used as an impedance inverter 12 are low pass filter networks of either a T or 1r configuration, and more particularly it has been found advantageous to use a two pole filter network such as illustrated in FIGURE 7 exhibiting a Chebyshev or Butterworth frequency response. The two pole filter network comprises two inductors 72 and 76 and two capacitors 74 and 78. The inductor 72, the capacitor 74 and the inductor 76 are connected in series between terminals 21 and 25 while the capacitor 78 is connected to the common connection between capacitor 74 and the inductor 76. The opposite end of the capacitor 78 is coupled to the common connection be- 4 tween terminals 22 and 24. Whether the two pole filter network illustrated exhibits a Chebyshev or Butterworth response is dependent upon the numerical values given to the inductor-capacitor combination.

With reference to FIGURE 2, the output impedanceof the solid state transmitter 10 at the terminals 21 and 22 is designated as Z and the load impedance comprising the circuit combination of the impedance inverter and the narrow band antenna system 14 is designated 2 In operation, the transmitter system is made to exhibit a broadband frequency characteristic utilizing a narrow band antenna system by controlling the output impedance Z looking back into the solid state transmitter substantially equal to the equation: x= /Q/2 where.

x=Z /Z and Q is a figure of merit of the antenna and is equal to the ratio of the reactance divided by the resistance of the antenna. Whereas prior art apparatus used impedance inverter networks to match the antenna to the transmitter, the present invention deliberately mismatches the transmitter to the load.

The manner in which the transmitter output impedance Z is controlled is shown more fully in FIGURE 3. The solid state transmitter 10 includes a power amplifieroutput stage comprising transistors 52 and 54 driven from a signal source, not shown, coupled to the input terminals 62 and 63 of the output transformer 50. The signal is applied across the primary winding 65 and drives transistors 52 and 54 by means of secondary windings 66 and 67 coupled to the base electrodes of transistors 52 and 54 respectively. A power supply 45 is shown connected to the output stage to supply power thereto and a capacitor 56 is connected across the power supply to act as a RF by-pass capacitance for the power supply. By operating the transistors 52 and. 54in the saturation region of their current-voltage characteristic, the output impedance Z can be made very low. In actual practice, when using transmitters composed of solid state devices in the switching mode, i.e., driving the devices alternately between their non-conducting and saturated states,the value of the ratio of the load impedance to the output impedance is often much higher than is desired in order to satisfy the equation: x= /Q/ 2. FIGURE. 3 illustrates that the output impedance Z can be controlled by placing a resistance 58 between the output stage and the output. terminal 21.

'FIGURE 3 further illustrates also that for practical applications the induct-ance of the antenna 38, the inductance of the loading coil 36, and the inductor 32 in the impedance inverter 12 can be combined into a single inductor 35.

FIGURE 4 illustrates another embodiment for controlling the output impedance Z of the solid state transmitter 10 looking back into the transmitter from output terminals 21 and 22. A small valued resistor 59 is coupled between the power supply 45 and the power output amplifier comprising transistor 52 and 54. The resistor 59, moreover, is connected between the power supply and the RF bypass capacitor 56 of the semiconductor amplifier,

thus allowing the collective voltage to drop when heavy region when using transistors. This can result in destruc-v tion of the transistor. By placing resistor 59 in the collector supply voltage instead of in series with the RF output, as shown in FIGURE 3, the collector voltage drops as the collector current increases providing safer operation of the transistors 52 and 54.

Another embodiment for controlling the output impedance Z of the solid state transmitter is shown in FIG- URE 5. In FIGURE 5, a low pass filter network 68 is coupled between the power supply 45 and the collector circuit of the power output amplifier comprising transistors 52 and 54. Such a network, having a impedance facing the collectors of the semiconductor amplifier that varies with frequency, will cause the transmitter 10' to have an output impedance Z as far as amplitude modulation is concerned, as though it was followed by an RF network having a response similar to the power supply network but symmetrical about the frequency of the carrier. FIGURE 6 is illustrative of a typical low pass filter network comprising resistor 59, conductor 60 and capacitor 57. A particular use anticipated with the solid state transmitter 10' is in the amplification of signals involving sudden phase reversals. Such a signal has a spectral distribution of (sinX)/X if there are random reversals at the clock rate. By placing a filter 68 between the power supply 45 and the semiconductor power output amplifier stage of such a nature that the impedance seen looking back into the filter from the amplifier is low at DC. and low frequencies, but high at frequencies above 2/ T, where T is the clock period, all sidebands further away from the carrier than the first null of the (sinX)/X distribution will be greatly attenuated. This not only results in less interference to other services, but also greatly reduces the peak current requirements when the transistors 52 and 54 used in the amplifier. A further advantage gained from placing such a filter in the collector supply 45 is as follows: where the only modulation is amplitude modulation, the efiiciency can be kept high at the carrier conditions by using a large value of x, where x=Z /Z This will of course result in a double hump curve for many values of antenna Q, similar to the curve obtained by an overcoupled circuit. A filter placed between the power supply and the amplifier could be used to flatten the top of the response curve for a portion of the bandwidth and result in less loss than the use of a resistor 59 along and at the same time provide as great a reduction of the peak currents in the semiconductors 52 and 54.

What has been shown, therefore, is a means for increasing the bandwidth characteristic of a transmitter system operating at relatively low operating frequencies into a narrow band antenna system while maintaining high efiiciency.

While there has been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, but it is to be understood that all equivalents, alterations, and modifications within the scope and spirit of the present invention are meant to be included.

I claim as my invention:

1. Apparatus for increasing the bandwidth of a radio frequency transmitter system comprising in combination: a series fed antenna system including an electrically short narrow band antenna and means for resonantly tun-ing said antenna to a predetermined output frequency; an impedance inverter network coupled to said antenna system; a radio frequency transmitter, having a controlled output impedance, coupled to said impedance inverter for feeding electrical energy to said antenna system; and means for controlling said output impedance so that the ratio of the load impedance provided by the combination of said impedance inverter and said antenna system to said output impedance is substantially equal to the equation Q7, where Q is the ratio of the reactance to the resistance of said antenna system.

2. Apparatus for increasing the bandwidth of a radio frequency transmitter system comprising in combination: an antenna system, having a predetermined resistive impedance, and including an electrically short narrow band antenna and a means for resonantly tuning said antenna to a predetermined output frequency; an impedance inverter network coupled to said antenna system and having a characteristic impedance substantially equal to said resistive impedance of said antenna system; a radio frequency transmitter, having a controlled output impedance, coupled to said impedance inverter for feeding electrical energy to said antenna system; and means for controlling said output impedance so that the ratio of the impedance combination of said impedance inverter and said antenna system to said output impedance is substantially equal to /Q/2 where Q is the ratio of the reactance to the resistance of said antenna system.

3. A wide band high efficiency solid state transmitter system comprising in combination: an antenna system including an electrically short, narrow band, antenna and a means for resonantly tuning said antenna to a predetermined output frequency; an impedance inverter network coupled to said antenna system; a solid state radio frequency transmitter having a semiconductor power output stage and output terminals; and resistive means coupled between said output stage and said output terminals for controlling said output impedance of said transmitter so that the ratio of the load impedance provided by the combination of said impedance inverter and said antenna system to said output impedance is a predetermined fraction.

4. A wide band, high efiiciency solid state transmitter system comprising in combination: an antenna system including a narrow band antenna and a means for resonantly tuning said antenna to a predetermined output frequency; an impedance inverter network coupled to said antenna system; a radio frequency transmitter adapted to be connected to said power supply and having a power output stage comprised of semiconductor devices for feeding electrical energy to said antenna system; and means for controlling said output impedance of said transmitter so that the ratio of a load impedance, as seen from said transmitter, and comprising the combination of said impedance inverter and said antenna system, to said output impedance looking back into said transmitter is substantially equal to /Q/2 where Q is the quality factor of said antenna system.

5. The Wide band high efficiency transmitter system according to claim 4, wherein said means for controlling said output impedance comprises resistor means coupled between said power supply and said power output stage.

6. The wide band high efficiency transmitter system according to claim 4, wherein said means for controlling said output impedance comprises a low pass filter network coupled between said power supply and said power output stage.

7. A wide band high efficiency solid state transmitter system comprising in combination: an antenna system having a predetermined impedance, including an electrically short, narrow band, antenna and a loading coil for resonantly tuning said antenna to a predetermined output frequency; a phase shift network having a surge impedance substantially equal to the resistive portion of said predetermined impedance of said antenna system, coupled in series to said antenna system; a solid state radio frequency transmitter having a controlled output impedance, coupled to said 90 phase shift network for series feeding electrical energy to said antenna system; and means for controlling said output impedance so that the ratio of the impedance of the combination of said 90 phase shift network and said antenna system to said controlled output impedance is substantially equal to the equation, x= /Q/ 2 where x is the ratio of the impedance provided by said antenna and said loading coil 7 to said output impedance and Q is the quality factor of said antenna system and is equal to the reactance divided by the resistance of said antenna system.

References Cited UNITED STATES PATENTS 1,998,322 4/1935 Kaar 343-860 X 2,111,743 3/1938 Blurnlein et a1 343-86O 2,194,180 3/1940 La SablOniere 333-32 X 8 Parker 325-460 X Leeds 333-32 X Yoshii et al 330-18 X Sharrna 33018 X Swanson 325-179 X Shimada et a1 330-48 JOHN W. CALDWELL, Acting Primary Examiner.

DAVID G. REDINBA'UGH, Examiner. 

1. APPARATUS FOR INCREASING THE BANDWIDTH OF A RADIO FREQUENCY TRANSMITTER SYSTEM COMPRISING IN COMBINATION: A SERIES FED ANTENNA SYSTEM INCLUDING AN ELECTRICALLY SHORT NARROW BAND ANTENNA AND MEANS FOR RESONANTLY TUNING SAID ANTENNA TO A PREDETERMINED OUTPUT FREQUENCY; AN IMPEDANCE INVERTER NETWORK COUPLED TO SAID ANTENNA SYSTEM; AN RADIO FREQUENCY TRANSMITTER, HAVING A CONTROLLED OUTPUT IMPEDANCE, COUPLED TO SAID IMPEDANCE INVERTER FOR FEEDING ELECTRICAL ENERGY TO SAID ANTENNA SYSTEM; AND MEANS FOR CONTROLLING SAID OUTPUT IMPEDANCE SO THAT THE RATIO OF THE LOAD IMPEDANCE PROVIDED BY THE COMBINATION OF SAID IMPEDANCE INVERTER AND SAID ANTENNA SYSTEM TO SAID OUTPUT IMPEDANCE IS SUBSTANTIALLY EQUAL TO THE EQUATION VQ/2, WHERE Q IS THE RATIO OF THE REACTANCE TO THE RESISTANCE OF SAID ANTENNA SYSTEM. 